Method of making a billet suitable for manufacturing into a superconductor

ABSTRACT

A plurality of rods are assembled in a predetermined configuration to form a core which is surrounded by a molten matrix metal within a heated crucible. The temperature of the thusly charged crucible&#39;&#39;s upper portion is maintained above the matrix metal&#39;&#39;s melting point. In this respect, as the heat is applied to the top of the melt, the crucible is maintained in a hot environment while the bottom of the crucible is centrally chilled. In this manner the charge is solidified from the bottom toward the top so that the solidification progresses upwardly and outwardly in a conical pattern. After the controlled solidification is completed the casting is separated from the crucible to form a cored extrusion billet. In one embodiment the rods are separated from the casting and the resulting open holes are filled with superconductive material to form a composite superconductor extrusion billet. In another embodiment the rods themselves are made of a superconductive material so as to eliminate the step of separating the rods from the casting.

nite States Raymond et al,

[ Feb. 26, 1974- Inventors: .lan W. Raymond; Cy N.

Whetstone, both of Denver, Colo.

[73] Assignee: Cryomagnetics Coration,

Denver, Colo.

[22] Filed: June 18, 11970 [21] Appl. No.: 47,390

[52] US. Cl 1164/57, 29/5277, 29/599, 164/105, 164/112, 164/127 [51]Int. Cl. 822d 27/20, 822d 19/00 [58] Field of Seareh..... 29/599, 527.7;164/98, 100, 164/103,105,108,110,112,57

[56] References Cited UNITED STATES PATENTS 2,362,875 ll/1944 Zahn164/132 3,264,697 8/1966 Price et al 164/75 X 3,401,738 9/1968 164/3533,513,537 5/1970 29/599 2,792,605 5/1957 164/112 3,509,622 5/1970Bernert et al 29/599 Primary ExaminerJ. Spencer Overholser AssistantExaminer.lohn E. Roethel Attorney, Agent, or Firm-Griffin, Branigan andButler A plurality of rods are assembled in a predeterminedconfiguration to form a core which is surrounded by a molten matrixmetal within a heated crucible. The temperature of the thusly chargedcrucibles upper portion is maintained above the matrix metals meltingpoint. In this respect, as the heat is applied to the top of the melt,the crucible is maintained in a hot environment while the bottom of thecrucible is centrally chilled. In this manner the charge is solidifiedfrom the bottom toward the top so that the solidification progressesupwardly and outwardly in a conical pattern. After the controlledsolidification is completed the casting is separated from the crucibleto form a cored extrusion billet. In one embodiment the rods areseparated from the casting and the resulting open holes are filled withsuperconductive material to form a composite superconductor extrusionbillet. In another embodiment the rods themselves are made of asuperconductive material so as to eliminate the step of separating therods from the casting.

8 Claims, 5 Drawing Figures PATENTEDFEBZSISH SHEET 2 [IF 2 FURNACE HEATTEMP. CONTROL- F IG. 4

' INVENTORS JAN W. RAYMOND CLAY N. WHETSTONE ATTORNEYS METHOD OF MAKINGA BILLET SUITABLE FOR MANUFACTURING INTO A SUPERCONDTJCTOR BACKGROUND OFTHE lNVENTlON The present invention relates to a method and apparatusfor producing pattern-cored extrusion billets for compositesuperconductors; and to the production of particularly high qualitysuperconductor wire.

Composite superconducting wire is comprised of strands ofsuperconductive material inbeded in a matrix of a metalsuch as copperand is frequently extruded from a billet. One form of superconductivewire includes upward to 1000 filaments of superconductor alloydistributed in an array in a normal metal matrix such as copper. Thesefilaments should be continuous from one end of the wire or strip to theother and separated from one another over their entire length by thematrix material. Such composite structures are currently fabricated bytechniques involving extrusion of composite billets that aresubsequently swaged, drawn, or rolled into superconductive wire, rod orstrips which form the final product. Each billet used in the extrusionprocess is conventionally in the form of a right circular cylindricalmatrix of normal metal such as copper in which rods of a superconductingmaterial are arranged with their long axes parallel to the longitudinalaxis of the matrix cylinder. In this respect, it is customary to formsuch a billet by drilling holes in a copper slug and then loading theholes with a superconductive material.

Alternatively, a plurality of wafers of the matrix metal have holesdrilled therein in a predetermined pattern. The wafers are then stackedup so that their holes are aligned for receipt of superconductive rods.This wafer-rod structure is then encased to form an extrusion billet.Still another method of formingsuch billets is to form apattern-arranged bundle of rods comprised of the matrix material andsuperconductive materials. This pattern arranged bundle of rods is thenencased to form either an extrusion billet or a smaller bundle that canbe swaged or drawn into a final product without employing the extrusionstep. A method of this type is described in U.S. Pat. Nos. 3,465,429 and3,465,430 to Barber et al.

All of the above methods of producing composite superconductor materialshave certain drawbacks. For example, there is considerable waste andexpense involved in drilling holes in the matrix metal; and it is alsoexpensive to properly encase the matrix and rods. Also, particularly inconnection with the wafer-rod structure, there is a tendency for thesuperconductive rods to break while they are being reduced to finalform. Similarly, where it is necessary to encase the billet elements,the casing must be removed after extrusion or it frequently causesundesirable burrs or jagged protrusions on the finished wire. Hence,objects of this invention are to provide not only a more economical andreliable method and apparatus for producing superconductive wire, but toprovide a superconductive wire which itself is of a considerably higherquality than that obtained by prior art methods.

An intermediate object of the invention is to provide a superconductorextrusion billet that is substantially defect free. ln this regard it isa principle of the invention to form such a billet by means of a uniquecasting technique. Barber el al have suggested that extrusion billetscan be cast, but such techniques have not heretofore proven practical.One reason for this is the generally accepted belief that extremelyexpensive and sophisticated casting equipment would be necessary to makedefect free castings of the required geometry and size because of thelarge shrinkage tendencies and other defect producing mechanismsencountered during solidification of normal matrix metals.

in accordance with principles of the invention a cored billet matrix isformed in a manner similar to that sometimes used in connection withfuel elements for nuclear reactors. One such technique is described inan article by A.W. Hare and R.F. Dickenson appearing at p. 210 et seq.,Vol 66 of the Transactions of American Foundrymens Society 1958). inthis regard a plurality of rods are assembled in a predeterminedconfiguration to form a core which is surrounded by a controlled purityand composition molten matrix metal within a heated crucible. The top ofthe crucible and its charge are maintained above the matrix metal smelting point; and, at the same time, a chill block is brought intocontact with the bottom center ofthe crucible while the sides of thecrucible are maintained in a hot environment. In this manner, the chargesolidifies from the bottom up and the center out so that it solidifiesin the pattern of an upwardly progressing cone. in this mannershrinkage, porosity and other defects are eliminated so that theresulting matrix metal is nonporous throughout. Moreover, the castbillet is formed without sophisticated and complex zone refiningequipment, vibrational casting apparatus, centrifugal casting devices orthe like. Consequently, the method of the invention is relativelyinexpensive.

A major advantage of the invention is the provision of a coredsuperconductor billet having a unitary matrix metal structure. in thisregard, where a plurality of matrix metal rods are assembled in acontainer there is considerable difficulty in bonding the similar-metalrods together. Some writers hold that it is not necessary to obtaincomplete bonding between the matrix metal elements, but we have foundthat complete bonding is quite important; and one of the reasons ourmethod results in a superior product is that the unitary matrix metalportion of our extrusion billet has no unbonded portions as will now bebriefly discussed.

When pattern arranged bundles of matrix metal rods and superconductorrods are placed in a container and co-reduced as in the method of Barberet al, the ductile but abrasive superconductor rods are almostinstantaneously bonded to their adjacent rods of normal matrix metal.Contiguous matrix metal rods, on the other hand, shift, slide, andreadjust under stresses of coreduction. They then transmit unevenstresses to the ductile superconductor filaments so that the resultingfilaments in the composite billet do not have uniform cross-sectionsthroughout their lengths. Moreover, if the rods of normal metal are notsubstantially absolutely clean they are even less adequately bonded andthe resulting filaments are even less uniform. This, in turn, places arestriction upon the critical current density of the superconductor wirethat is drawn or otherwise formed from the composite billet. Theproducts resulting from the method of the instant invention, on theother hand, have uniform superconductor filaments whereupon they exhibitsubstantially higher useful critical current densities thatcorresponding composite superconductors made by the above describedtechniques of the prior art.

One prior art technique for obtaining better bonding between the matrixmetal elements of a patterned-rod billet has been to co-reduce thebillet at a relatively high temperature. Such high temperatures,however, cause the normal metal surfaces to react with surroundingactive gases to severly reduce the bonding characteristics of the normalmetal elements and thus degrade the quality of the product. Hence, whenthis technique has been employed it has been necessary to initiallycoreduce such composite structures in a vacuum or an inert-gasatmosphere. This, however, is a cumbersome and expensive procedure whichis not required when the method of the invention is employed.

It is often desirable to use long lengths of composite superconductorwire rather than two or more shorter pieces; and, longer pieces of suchcomposite wire are more susceptible to undesirably low critical currentdensities because there is a greater probability that the longer wirewill have a filament defect somewhere along its length. Hence, it isanother object of this invention to provide a method of making longlengths of composite superconductor wire having high critical currentdensity ratings. In this regard, it has become conventional practice totwist composite superconductor material in order to increase its usefulcritical current density in magnet applications. When compositesuperconducting materials are thusly twisted, however, normal metalbonding defects are magnified and thus degrade the full benefit of thetwisting step. One of the advantages of the instant invention,therefore, lies in the ability of its resulting wire to be twisted so asnot to destroy the filament integrity and thus obtain the full benefitsof the critical current density increases due to the twist.

Another object of the invention is to provide a method of easily,accurately, and economically controlling the resistivity ratio ofcomposite superconductive wire; and in accordance with the principles ofthe invention dealing with this particular object, an alloying orcontamainant" material is added to the molten matrix metal in an amountcorresponding to a predetermined resistivity ratio of the correspondingsuperconductor.

And finally, in accordance with further aspects of the invention, afterthe casting has been solidified as described above the coi'ed casting isseparated from the crucible so that the rods can be removed if desired.In this event the resulting holes can be filled with a superconductivematerial to form an extrusion billet which is drawn or otherwisesuitably reduced to form superconductive wire, rod, or strip. In thismanner the resulting product not only has operating superconductingcharacteristics that are far superior to those produced by prior artmethods, but the wire is free of burrs or jagged protrusions resultingfrom outer containers such as those described in US. Pat. 3,465,430.Hence the smooth surface obtained by this invention is easier to bothfabricate and insulate.

BRIEF DESCRIPTION OF THE DRAWINGS The foregoing and other objects,features, and advantages of this invention will be apparent by thefollowing more particular description of preferred embodiments thereofas illustrated in the accompanying drawings wherein the same referencenumerals refer to the same parts throughout the various views. Thedrawings are not necessarily intended to be to scale, but rather arepresented so as to illustrate the principles of the invention in clearform.

In the drawings:

FIG. 1 is an end view of a billet core used in an embodiment of theinventions method;

FIG. 2 is a sectional view of the FIG. 1 billet core taken along thelines 22 of FIG. 1.

FIG. 3 is a schematic view of a portion of a furnace and crucibleadapted to practice a preferred embodiment of the inventions method;

FIG. 4 is a schematic illustration of the manner in which a billetmatrix is solidified during a directionally controlled freezing step ofthe methods preferred embodiment; and,

FIG. 5 is a schematic illustration of apparatus for controlling thesolidification of the billet matrix illustrated in FIG. 4.

DETAILED DESCRIPTION In the embodiment of the core array illustrated inFIGS. 1 and 2, a retainer plate is affixed to both a core support rod 12and a lower pattern plate 14. These elements are made of a high puritygraphite.

The lower pattern plate 14 and a corresponding upper pattern plate 16have holes 18 drilled therein to accommodate the ends of a plurality ofrods 20 preferably hollow quartz which have at least one end sealed asat 22 to prevent the escape of air and the entrance of copper during animmersion step to be described shortly.

The core array of FIG. 1 is assembled by screwing the graphite coresupport rod 12 into the lower rod retainer plate 10, sliding the lowerpattern plate 14 down the core support rod, and sliding the upperpattern plate 16 into its proper position on the core support rod. Boththe upper and lower pattern plates are than locked in position withsmall tungsten or graphite pins such as 26 and 28. Next, a suitablenumber of quartz tubes 20 are loaded in the partially assembled core byinserting them, closed end 22 up (to the right in FIG. 2) through theholes 18 in the top pattern plate 16 so that they terminate incorresponding holes in the bottom pattern plate 14. Subsequent tocompletely installing the appropriate rod pattern, the top retainerplate 24 is slid down the core support rod 12 and securely pinned asdescribed above.

After the core array of FIG. 1 is constructed as described above, it isplaced in a furnace to be heated. At the same time, a crucible 30 (FIG.3) is placed on a somewhat donut-shaped stool 32 in a furnace 34. Inthis regard, the inside walls of the crucible are slightly tapered at arate of about one quarter inch per foot so that the crucibles insidediameter is smaller at its bottom end than its top which is covered byan insulated plug 35. This graphite plug 35 is machined to both fit thetop of the crucible and accommodate a tube 36 for delivering inert gasto the crucibles interior if desired.

The furnace has one or more primary heat inlets such as 38; one or moresecondary heat inlets such as 40; and a hole 42 in the bottom thereof toaccommodate a chill-block 44 which may be raised upwardly through thestool 32 to rest against the bottom of the crucible 30. A temperaturesensing element 46 enters the top of the crucible and passes along itsside to sense the temperature of the crucible at various points alongits length. A second temperature sensing element 48 extends up to thecrucibles bottom adjacent to the chillblock 44; and both of thetemperature sensing elements are connected to a temperatureindicator-controller 50 (see also H6. 5).

The temperature indicator controller 50 provides outputs on lines 52 and54 to control primary and secondary heaters 56 and 58 respectively whichprovide heat to the primary and secondary heat inputs 38 and 40 to thefurnace as shown in H6. 5. Similarly, the temperature control 50provides an output on line 58 for controlling a. chill water supply 60which delivers chill water through conduit 62 to a chill water'recess 64in the furnaces chillblock 44. The heaters and chill water supply canalso be controlled manually.

in practicing a preferred embodiment of the inventions method a chargeof high purity oxygernfree, highconductivity (OFHC) copper is placed inthe crucible 30 as illustrated by dotted line 65 in W6. 3. In thisregard, the crucible 30 is preferably of a high purity graphite in orderto minimize melt contamination from this source. This is particularlyimportant where the inventions method is used to produce superconductivewire having a high resistivity ratio. That is, the ratio of itsresistance at room temperature to its resistance at the superconductivetemperature such as 4.2K., for example. Typically desired resistivityratios are about l50-200, but this ratio drops rapidly as impurities areintroduced into the copper. To this end, the use of graphite issignificant because it does not combine with copper, but the cruciblecan also be made out of other materials which would not contaminate thecopper. Similarly, matrix materials other than copper can also be used;and, in those cases where it is desired to provide a superconductorhaving a low resistivity ratio the copper or other matrix can beintentionally alloyed. For example, as little as 7 percent nickle dropsthe resulting structures resistivity ratio to about 5:1. in this regard,it has been found that the method of the invention is admirably suitedfor both accurately and inexpensively controlling the resistivity ratioof composite superconductive wire. For any given combination of normalmetal and superconductor metal the composite wires resistivity ratio canbe controlled to within an accuracy that has not been previouslyobtainable in commercially available composite wire.

After the charge has been melted and superheated, it fills the crucibleto about the level of line 66 in H6. 3. At this point, the preheatedcore assembly is slowly lowered into the melt so as to be covered bymolten copper. The primary heater is then turned off and the melt issubjected to a controlled unidirectional freezing step which will now bedescribed.

The temperature controller 50 is adapted to direct chill water fromsource so through conduit 62 to the chill-water cavity 64, of thechill-block 44.. At the same time, heat from source 58 is directedthrough secondary heating conduits 40 toward the upper portion of thecrucible so that the top of the melt is maintained above the matrixmetals melting point 1083" centrigrade in the case of ()FHC copper; andthe crucible is kept in the furnace so that its sides, although notfurther specifically heated, are maintained in a hot environment. inthis manner, the matrix gradually solidifies from the bottom up andinside out in a cone-like fashion so as to eliminate the casting defectsgenerally associated with uncontrolled soldification. For example, withthe controlled solidification step a shrinkage cavity does not form inthe center of the billet as occurs if the melt solidifies from theoutside in.

In the above regard, it has been found that by simultaneouslycontrolling the chilling of the crucibles lower portion and heating itsupper portion during the solidification process a pyramid or cone typesolidification pattern results. That is, as illustrated in FIG. 4, themelt 68 solidifies first at its bottom center and then at its outeredges in the manner of an upwardly progressing pyramid or cone placed ontop of the previously solidified matrix below. For example, dotted-line70 might represent the extent of solidification at a first point intime; dotted line 72 might represent the extent of solidification at asubsequent point in time; and dotted-line 74! might represent the extentof solidification at a still later point in time. It is thissolidification cone pattern of progressive solidification that providesa casting that is substantially free of undesired voids or conventionalcasting defects.

After the casting is sufficiently cooled it is extracted from thecrucible with the core array cast inside. The portions of the castingcontaining the pattern and retainer plates are then cut off and, ifdesired, the outer surface of the casting is turned to the desireddimensions. in this regard, however, one of the advantages of theinvention is that only a small portion of the originally cast matrixmaterial is wasted. For example, the

above described steps of turning and removing the end and retainerplates involves removing only about 5 percent of the matrix materialwhich, because of its high purity can be reused. Note, in this regard,that if holes are drilled in either a unitary structure or individualwafers it is quite difficult to maintain the purity of the thuslyremoved material, whereupon it is not satisfactory for subsequent use asa superconductive matrix.

If desired the quartz rods are next removed from the casting by aleaching process in which the quartz is dissolved under the action ofmolten sodium hydroxide. The hot billet is then water-quenched to ensureremoval of any undisolved quartz and to minimize oxidation of the billetsurface. Alternatively, the quartz rods can be removed by leaching inhydrofluoric acid solution or by mechanical means. After this the matrixcasting is cleaned by brushing, leaching, and washing in suitablereagents to provide clean active surfaces. The matrix holes are thenfilled with rods of superconductive elements or alloys such as niobium,or some appropriate superconducting alloy. For example, satisfactoryresults can be obtained by using a niobiumtitanium alloy having up to 70percent titanium (titanium 30 weight percent niobium); and in apreferred embodiment the composite billet consisted of titanium 45weight percent niobium rods embedded in an OFHC copper matrix. Whicheverthe case the composite billet is then extruded, swaged, drawn or in someother way fabricated into composite superconductive wire, rod or strip.

it will be appreciated by those skilled in the art that the abovedescribed method and apparatus for producing a uniform matrix alsoprovides a uniform, superior quality superconductive wire. in thisregard, not only is such wire more uniform, but there are far lessincidents of filament damage than in wires made by prior art processes.Consequently, the resulting wire can be drawn into much longer lengthswithout suffering a re duction in useful critical current density. Forexample, a comparison was made between composite wire made by aconventional method and composite wire of the same diameter andcomposition, but made by the above described method. The maximum lengthof high quality composite 0.050 diameter wire made by the conventionalmethod was 4,000 feet, while high quality 0.050 wire of the instantinvention was drawn to 40,000 feet and it could have been longer ifdesired.

Also, the surface of the resulting wire is free of burrs as opposed tothat of prior art processes because the composite extrusion billet isnot segmented and thus it is not necessary that the billet be placed ina container prior to drawing. Hence, there are no undesirableprotrusions or burrs in the final wire so that it is considerably easierto insulate. It should also be noted that when superconductive wiresmade in accordance with the invention are placed in structures such assuperconductive magnets, they result in a magnet that is much easier toenergize because it is both easier to insulate in a short-free manner;and capable of obtaining a higher flux density because of its filamentintegrity.

While the invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention. For example, it will be appreciated that the above describedcentral graphite supporting rod could be a supporting circumferencialsleeve or longitudinal straps, the quartz tubes could be drilled out;and the inventions method can also be applied to continuous casting.Also, other materials can be used than those specifically described. Forexample, rods of stainless steel or other suitable metals can be coatedwith AlQ IiO ,gr other refraclory compounds, and such structures can beused in place of the quartz rods described above; and, if the rods aretapered, they can be removed mechanically;

The configuration of the crucible mold, core components and billet canalso be changed markedly without departing from the spirit of theinvention. For example, square or hexagonal cross sectioned billets canbe produced; and optimum packing factors may make it desirable to usecore rods having hexagonal, triangular or other geometrical shapes.

The quartz rods can also be replaced with superconductor rods whosesurfaces have been treated with niobium, molybdenum, tungsten, or thelike so as not to combine with the matrix metal and/or form compoundsdetrimental to the fabrication and useful current density of thesuperconducting end product.

In this regard the invention can be practiced by placing core rodscomposed of a superconducting material directly in the core assembly toproduce a finished extrusion billet as the cast product rather than acored extrusion matrix that must subsequently be loaded with asuperconductive material to form the finished composite extrusionbillet. For example, Ti-Nb core rods can be directly cast in an aluminummatrix. Also, instead of inserting the core assembly into the moltenmatrix metal, the core can be fixed in the crucible and the moltenmatrix metal introduced from an outside source; and, in this respect, itwill be appreciated that other matrix metals such as lead and tin canalso be used. Hence, it Wlll be apparent that the invention can bepracticed in many manners other than those which have been specificallydescribed above.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

l. A method of making a cast extrusion billet suitable for manufacturinginto a composite superconductor comprising the steps of:

creating a melt of matrix metal in a form;

adding to said melt an alloy material in a predetermined amountcorresponding to a predetermined resistivity ratio of the resultingcomposite superconductor;

locating an array of preheated rods in said melt such that the meltenvelopes the array of rods;

sensing the temperature of said form at predetermined points along thesurface thereof running from the bottom to the top;

controlling the solidification of said melt from the bottom of saidbillet toward the top thereof in accordance with said sensed temperaturein a manner such that said solidification progresses upwardly in thepattern of a cone so as to eliminate undesired voids and casing defects,and obtain a nonporous uniform billet; and,

separating the solidified matrix and said array from said form toprovide a cast extrusion billet suitable for manufacturing into saidcomposite superconductor.

2. The method of claim 1 wherein said controlled solidification includesthe step of:

cooling the melt at the bottom while:

maintaining said form in a hot environment above the melting point ofsaid matrix metal; and,

heating said form at the top of said melt to maintain the temperaturethereof above the melting point of said matrix metal until the top ofsaid melt solidifies.

3. The method of claim 1 wherein said rods are comprised of asuperconductor material and wherein said controlled solidificationincludes the step of:

cooling the melt at the bottom while:

maintaining said form in a hot environment above the melting point ofsaid matrix metal; and, heating said form at the top of said melt tomaintain the temperature thereof above the melting point of said matrixmetal until the 4. The method of claim 1 wherein said rods are quartz.

5. The method of claim 4 including the step of:

cooling the melt at the bottom while:

maintaining said form in a hot environment avobe the melting point ofsaid matrix metal; and,

heating said form at the top of said melt to maintain the temperaturethereof above the melting point of said matrix metal until the top ofsaid melt solidifies.

6. The method of claim 1 wherein said rods are of refractory-compoundcoated metals.

7. The method of claim 1 wherein said rods are comprised ofsuperconductive metal surface-treated with metals selected from thegroup consisting of niobium, molybdenum, and tungsten.

8. A method of making an extrusion billet suitable for manufacturinginto a composit superconductor comprising the steps of:

constructing an array of parallel rods in a frame so that said rods arearranged in a pattern about a central longitudinal axis;

preheating said array of parallel rods and said frame;

locating a matrix metal in a crucible and heating said matrix metal insaid crucible to a molten state;

adding to said molten matrix metal an alloy material in a predeterminedamount corresponding to a predetermined resistivity ratio of theresulting composit superconductor;

inserting said preheated array of parallel rods and said frame in saidmolten metal matrix in said crucible;

sensing the temperature of said crucible at predetermined points fromthe bottom to the top thereof;

solidifying said molten matrix metal about said array of parallel rodsfrom the bottom of said crucible separating the thusly formed billetfrom said crucible.

1. A method of making a cast extrusion billet suitable for manufacturinginto a composite superconductor comprising the steps of: creating a meltof matrix metal in a form; adding to said melt an alloy material in apredetermined amount corresponding to a predetermined resistivity ratioof the resulting composite superconductor; locating an array ofpreheated rods in said melt such that the melt envelopes the array ofrods; sensing the temperature of said form at predetermined points alongthe surface thereof running from the bottom to the top; controlling thesolidification of said melt from the bottom of said billet toward thetop thereof in accordance with said sensed temperature in a manner suchthat said solidification progresses upwardly in the pattern of a cone soas to eliminate undesired voids and casing defects, and obtain anonporous uniform billet; and, separating the solidified matrix and saidarray from said form to provide a cast extrusion billet suitable formanufacturing into said composite superconductor.
 2. The method of claim1 wherein said controlled solidification includes the step of: coolingthe melt at the bottom while: maintaining said form in a hot environmentabove the melting point of said matrix metal; and, heating said form atthe top of said melt to maintain the temperature thereof above themelting point of said matrix metal until the top of said meltsolidifies.
 3. The method of claim 1 wherein said rods are comprised ofa superconductor material and wherein said controlled solidificationincludes the step of: cooling the melt at the bottoM while: maintainingsaid form in a hot environment above the melting point of said matrixmetal; and, heating said form at the top of said melt to maintain thetemperature thereof above the melting point of said matrix metal untilthe
 4. The method of claim 1 wherein said rods are quartz.
 5. The methodof claim 4 including the step of: cooling the melt at the bottom while:maintaining said form in a hot environment avobe the melting point ofsaid matrix metal; and, heating said form at the top of said melt tomaintain the temperature thereof above the melting point of said matrixmetal until the top of said melt solidifies.
 6. The method of claim 1wherein said rods are of refractory-compound coated metals.
 7. Themethod of claim 1 wherein said rods are comprised of superconductivemetal surface-treated with metals selected from the group consisting ofniobium, molybdenum, and tungsten.
 8. A method of making an extrusionbillet suitable for manufacturing into a composit superconductorcomprising the steps of: constructing an array of parallel rods in aframe so that said rods are arranged in a pattern about a centrallongitudinal axis; preheating said array of parallel rods and saidframe; locating a matrix metal in a crucible and heating said matrixmetal in said crucible to a molten state; adding to said molten matrixmetal an alloy material in a predetermined amount corresponding to apredetermined resistivity ratio of the resulting compositsuperconductor; inserting said preheated array of parallel rods and saidframe in said molten metal matrix in said crucible; sensing thetemperature of said crucible at predetermined points from the bottom tothe top thereof; solidifying said molten matrix metal about said arrayof parallel rods from the bottom of said crucible toward the top thereofin accordance with said sensed temperature in a manner such that saidsolidification progresses upwardly in the pattern of a cone by coolingthe crucible at the bottom while: maintaining said crucible in a hotenvironment above the melting point of said molten matrix metal; and,heating said crucible at the top to maintain the temperature thereofabove the melting point of said molten matrix metal until the top ofsaid melt solidifies; and, separating the thusly formed billet from saidcrucible.